Airborne vehicle nap-of-the-earth (NOE) flight requires precise pilot control to avoid obstacles and elevated terrain. While the pilot normally relies on good visibility to perform the NOE function, requirements exist for NOE flight during periods of less than good visibility (poor weather or night time conditions) and (pilotless) automatic guidance vehicles. Airborne navigation systems are typically of the dead reckoning (DR) variety, usually based upon doppler radar and compass systems; while these navigation schemes offer excellent short term guidance capability, they exhibit unacceptable long term position error growth, so that an in-flight correction procedure, such as periodic position updating of the dead reckoning navigation system, is necessary. Unfortunately, currently available radio aids (even assuming their presence in the vehicle's operational area) are generally insufficient to support NOE flight conditions where line-of-sight communications are not possible.
A proposal to solve this problem has been the concept of employing terrain correlation techniques through which the position of the aircraft as determined by its navigation system is updated as a function of the overflown terrain, its elevation heights and height variations. Examples of such correlation schemes include Terrain Contour Matching (TERCOM) which relies on elevation profile correlations and Sandia Inertial Terrain Aided Navigation (SITAN) which employs slope information to accomplish vehicle navigation. (For a discussion of navigation systems, including TERCOM and SITAN navigation schemes, attention may be directed to articles entitled "Continuous Kalman Updating of an Inertial Navigation System Using Terrain Measurements" by R. D. Andreas et al, Sandia National Laboratories pp. 1263-1270, IEEE 1978; "An Alternative Approach for Terrain-Aided Navigation Using Parallel Extended Kalman Filters" by T. C. Sheives et al, Sandia National Laboratories, Albuquerque, N.M., December 1979; and "Application of Multiple Model Estimation Techniques to a Recursive Terrain Height Correlation System" by W. Tang et al, pp. 757-764, IEEE 1981 and the U.S. Pat. Nos. to Evans et al 4,179,693; Webber 4,144,571; Thomas et al 4,103,847; Blatchford 3,992,613; Rawicz 4,320,287; Keearns 4,495,580; Sakaki et al 4,472,812; Fleishman 4,232,313; Graupe et al 4,185,168; Broniwitz et al 3,952,304; and Giles et al., 4,511,219; and the British patent to Fryen et al GB No. 2025723A.)
Within the above-referenced approaches the parallel filter system described in the Andreas et al article incorporates both TERCOM and SITAN signal processing mechanisms in an effort to improve upon navigation performance. In this system the basic navigation tool is the SITAN processor which requires that position information satisfy specific accuracy limitations. For this purpose an error decision algorithm is employed to monitor the error in the SITAN processor. When this error, which is expected to and permitted to expand, particularly during flight over monotonous terrain (e.g. water), reaches a prescribed magnitude, an attendant TERCOM signal processor (typically referenced as parallel filters) is engaged to update or correct the position information upon which the SITAN processor relies, thereby reducing the error and allowing the SITAN processing algorithm to proceed. In other words, the combined TERCOM-SITAN parallel filter scheme of Andreas et al is essentially a SITAN mechanism in which an adjunct TERCOM processor is engaged only when an error decision mechanism detects that the error in the SITAN processor is unacceptable. One undesirable aspect of this approach is the fact that the navigation mechanism effectively oscillates-it starts on course, is permitted to stray and is then pulled back and restarted. The resulting guidance of the aircraft generates a flight path that is a zig-zag route, on and off the intended flight path.
In addition, because the parallel filter SITAN-TERCOM scheme requires an error monitoring algorithm to determine if and when the TERCOM processor is to be engaged, it is computationally complex and inefficient.